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Gas furnace cycle on and off and then goes into lockout.

I have Coleman 95% gas forced hot air furnace (not a mobile home furnace, a full house furnace). We starting having problems a few weeks ago. It would run fine for 20 minutes or so and then loose flame and eventually go into lockout. So I called an HVAC company. The guy replaced draught inducer which didn't fix anything. He came back and cleaned the flame sensor. Then came back again and cleaned the condensate drain, still kept shutting down. I finally looked and the manual and showed him the error code that said the furnace was loosing flame five time during the cycle which was either low gas pressure or faulty gas valve. He ordered a new gas valve but when it came in he realized he ordered the wrong one. I fired him.

The next guy came and didn't even open the furnace just said it was 13 year old and we needed a new one he could install within a few days. I sent him home.

Third guy came and said he felt the pressure switch was sending shutdown signals so that is why we were loosing flame. He replaced the pressure sensor which did not solve anything.

Rather than calling someone else I got out my multimeter and set up shop in front of the furnace. Here is what I have observed.

When the furnace gets a call for heat, the draught inducer comes on as expected, after a few seconds the pressure switch reads 24V for a seconds and returns to 0V. When it reads 24V the ignitor comes on. With NO flame I am getting about 12V between the flame sensor and ground. A few seconds later the gas valve opens (reads 24V) and I get flame. All this time I am reading 0V from the limit switch. With the flame on I get about 40V on the flame sensor. Whitout any changes from the limit switch the gas valve will close and I loose flame. When I loose flame the inducer and main blower stay. The ignitor immediately come back on and eventually the gas valve opens and it will run fine for a while and then the whole thing repeats. After five flame outs the systems shuts down in lookout mode and I get the max five flame out error message.

I am not going to fix this myself, don't like working with anything that can blow me up. I just want to be able to communicate with the service tech because they seem to be guessing rather trying to actually test things.

1 question: Where is the 40V across the flame sensor coming from or is that normal? With only 24V circuit I can't figure that one out.

Any other suggestions on what I should tell the tech when he comes? Any suggestions on what he should look at?

The flame sensor is read in dc millivolts so what you are reading to ground tells me nothing. A competent tech should be able to find the problem in a few minutes. Keep trying til you find the right tech, if the problem is not fixed all the companies you've had out will probably come out for no additional charge, just ask for a different tech than the one they sent the first time.

Are you reading across the pressure switch? The circuit board looks for an open pressure switch before starting the draft inducer blower. You should not be reading 12V or 40V at the flame sensor. Without being there it's hard to diagnose the problem. Where are you putting your meter leads on the limit switch? As far as the tech is concerned don't tell him anything before he comes. Ask him before he comes if he has encountered this problem and can he fix it the first time out?